Field of the Invention
Embodiments of the present invention generally relate heart rate measurement and more specifically relate to pulse oximetry based heart rate measurement that is tolerant to sensor displacement.
Description of the Related Art
Heart rate measurement is an important problem in the context of many applications including medical monitoring, sports training, and fitness. Traditionally, heart rate monitoring has been performed in a static medical setting by trained professionals. However, recent trends in sports and fitness have created a demand for effective unsupervised heart rate monitoring in an uncontrolled environment. The demand for continuous monitoring suggests that the device used to measure the heart rate should be wearable and seamless. A watch-like form factor is suitable for these requirements since this form factor is a commonly used form factor for heart rate monitoring during sports or fitness training. Further, the use of light (visible or otherwise) as the sensing modality may have fewer limitations than traditional approaches like electrocardiogram (EKG) in a wrist mounted device as there may not be sufficient distance between the electrodes to obtain a good EKG signal.
Pulse oximetry is a commonly used noninvasive light-based technique for measuring the oxygen saturation level of arterial blood and pulse (heart rate). Typically, the sensor portion of a pulse oximeter passes light through blood perfused tissue, e.g., a finger or an ear lobe, and photoelectrically senses the absorption of light in the tissue. Arteries expand and contract due to blood flow and, thus, the amount of absorbed light changes in the course of a heart beat. The resulting signal is referred to as a photoplethysmograph (PPG). The PPG signal may be analyzed to determine, among other thing, the heart rate of the person to which the PPG sensor is attached.
Typical pulse oximetry techniques assume that the person to whom the PPG sensor is attached is relatively stationary and motionless. Thus, such techniques may not account for motion artifacts and relative displacement of the sensor when analyzing the PPG signal. Many approaches have been proposed to address motion artifacts in PPG signals including adaptive filtering with a reference of an additional sensor, e.g., an accelerometer, and different statistical learning models. Some such approaches are described in L. Wang, et al., “Multichannel Reflective PPG Earpiece Sensor With Passive Motion Cancellation,” Biomedical Circuits and Systems, IEEE Transactions on, Vol. 1, No. 4, pp. 235-241, December 2007, M. R. Ram, et al., “On the Performance of Time Varying Step-Size Least Mean Squares (TVS-LMS) Adaptive Filter for MA Reduction from PPG Signals,” Communications and Signal Processing (ICCSP), 2011 International Conference on, pp. 431-435, February 2011, and K. Naraharisetti, et al., “Comparison of Different Signal Processing Methods for Reducing Artifacts from Photoplethysmograph Signal,” Electro/Information Technology (EIT), 2011 IEEE International Conference on, pp. 1-8, May 2011. None of these approaches, however, address the issue of relative sensor displacement, i.e., changes in position or orientation of the sensor. In addition, the proposed approaches for removing motion artifacts need improvement.